This invention relates to modified isoprenoid synthase enzymes, their encoding genes, and uses thereof.
The term isoprenoid is used to refer to a family of compounds derived from the isoprene building block. In particular, plant isoprenoids comprise a structurally diverse group of compounds that can be divided into classes of primary and secondary metabolites (FIG. 1). Isoprenoids that are primary metabolites include sterols, carotenoids, growth regulators, and the polyprenol substituents of dolichols, quinones, and proteins. These compounds are essential for membrane integrity, photoprotection, orchestration of developmental programs, and anchoring essential biochemical functions to specific membrane systems, respectively. Isoprenoids that are classified as secondary metabolites include monoterpenes, sesquiterpenes, and diterpenes. These compounds are said to mediate important interactions between plants and their environment. For example, specific terpenoids have been correlated with plant-plant (Stevens, In: Isopentoids in Plants, Nes, W. D. Fuller, G., and Tsai, L.-S., eds., Marcel Dekker, New York, pp. 65-80, 1984), plant-insect (Gibson and Pickett, Nature 302:608, 1983), and plant-pathogen interactions (Stoessl et al., Phytochemistry 15:855, 1976).
The common denominator for this diverse array of compounds is their universal five-carbon building block, isoprene. The "biogenic isoprene rule" was employed to rationalize the biosynthetic origins of all terpenoids derived from isoprene (Ruzicka, Experientia 10:357, 1953). The polymerization of two diphosphorylated isoprene building blocks (e.g., IPP and dimethylallyl) generates geranyl diphosphate (GPP), a linear C10 intermediate that can be converted to cyclic or linear end-products representing the monoterpenes, or used in another round of polymerization. The addition of a third isoprene unit to GPP generates farnesyl diphosphate (FPP), which can also be converted to cyclic or linear products representing the sesquiterpene class. Continuing the polymerization and chemical differentiation cycle leads to the production of other classes of terpenoids named according to the number of isoprene building blocks leading to their biosynthesis, for example, the addition of a third IPP to FPP generates geranylgeranyl diphosphate (GGPP).
These polymerization reactions are catalyzed by prenyltransferases that direct the attack of a carbocation (an electron deficient carbon atom resulting from the loss of the diphosphate moiety of one substrate) to an electron-rich carbon atom of the double bond on the IPP molecule (FIG. 2). The electrophilic nature of these reactions is said to be unusual relative to more general nucleophilic condensation reactions, but this appears to be a common reaction among isoprenoid biosynthetic enzymes and especially those enzymes involved in catalyzing the cyclization of various isoprenoid intermediates (Gershenzon and Croteau, In: Lipid Metabolism in Plants, Moore, T. S., ed., CRC Press, Boca Raton, Fla., pp. 340-388). The enzymes responsible for the cyclization of GPP, FPP, and GGPP are referred to as monoterpene, sesquiterpene, and diterpene synthases or synthases, and represent reactions committing carbon from the general isoprenoid pathway to end products in the monoterpene, sesquiterpene, and diterpene classes, respectively.
Two important biochemical distinctions between the prenyltransferase and synthase reactions are illustrated in FIG. 2. The prenyltransferases catalyze carbon-carbon bond formation between two substrate molecules, whereas the synthases catalyze an intramolecular carbon-carbon bond formation. The prenyltransferases also catalyze reactions with very little variance in the stereochemistry or length of the ensuing polymer. Prenyltransferases differ in the length of the allyic substrates that can be accepted in initiating these reactions. The synthases are also substrate specific. However, diverse sesquiterpene synthases, for example, can utilize the same substrate to produce different reaction products.
The biosynthesis of isoprenoids such as cyclic terpenes is said to be determined by key branch point enzymes referred to as terpene synthases. The reactions catalyzed by terpene synthases are complex, intramolecular cyclizations that may involve several partial reactions. For example, the bioorganic rationale for the cyclization of FPP by two sesquiterpene synthases are shown in FIG. 3. In step 1, the initial ionization of FPP is followed by an intramolecular electrophillic attack between the carbon bearing the diphosphate moiety and the distal double bond to form germacene A, a macrocylic intermediate. Internal ring closure and formation of the eudesmane carbonium ion constitutes step 2. For tobacco 5-epi-aristolochene synthase (TEAS), the terminal step is a hydride shift, methyl migration, and deprotonation at C9 giving rise to 5-epi-aristolochene as depicted in step 3a. Hyoscyamus muticus vetispiradiene synthase (HVS) shares a common mechanism at steps 1 and 2, but differs from TEAS in the third partial reaction in which a ring contraction would occur due to alternative migration of an electron pair. In each case, a monomeric protein of approximately 64 kD catalyzes the complete set of partial reactions and requires no cofactors other than Mg.sup.+2.